A manner for long term preserving biologic samples (including microbe, cell, tissue, spermatozoon and so on) includes mixing the sample with a cryo-culture medium, distributing the mixture to a cryotube or a cryovial and then putting the cryotube into a liquid nitrogen tank for storage at −196° C. A cap of the cryotube and a tube body of the cryotube have different degrees of thermal expansion and contraction due to the structure and material difference therebetween, when the cryotube are immerged into the liquid nitrogen. As a result, the liquid nitrogen is liable to invade into the tube body of the cryotube, which not only contaminates the biologic sample, but a gas explosion may also occur due to rapid vaporization of the liquid nitrogen in the tube body of the cryotube when the cryotube is taken out from the liquid nitrogen tank.
A commercially available thermoplastic film which is made from polyethylene (PE) can be used in for the long term storage at the temperature of the liquid nitrogen. In use, the translucent tubular PE film is cut in a desired length, is put on the cryotube and then is heated directly by intense heat flame of a gas gun or a small torch gun. The PE film shrinks for being heated and tightly seals the cryotube, thereby preventing the problems of contamination, and gas explosion caused by the invasion of the liquid nitrogen. However, the sealing process is done by an experienced operator manually, which is time consuming and laborious. Furthermore, quality of the sealing process varies and the chance of failure in the sealing process is high, which is about 40% for one of the inventors of the present invention. Further, if the heating time is too long during the sealing process due to inexperience or neglect of an operator, the biologic sample in the cryotube may be destroyed due to the overheating, or even the cryotube may be damaged. Therefore, the thermoplastic film sealing process is waived for most of the cryotube storage, and thus the problems caused by the migration of the liquid nitrogen into the cryotube are not solved.